1. Field of the Invention
The inventions relates to a metallocene catalyst useful in preparing syndiotactic/isotactic block polyolefins. The catalyst comprises a bridged metallocene in which one of the cyclopentadienyl rings is substituted in a different manner from the other ring. The invention further includes a process of preparing syndiotactic/isotactic block polyolefins that comprises the use of the disclosed catalysts.
2. Description of the Prior Art
The present invention provides a process for polymerizing olefins having three or more carbon atoms to produce a polymer with a syndiotactic/isotactic stereochemical configuration. The catalyst and process are particularly useful in polymerizing propylene to form a syndiotactic/isotactic block polypropylene.
As known in the art, syndiotactic polymers have a unique stereochemical structure in which monomeric units having enantiomorphic configuration of the asymmetrical carbon atoms follow each other alternatively and regularly in the macromolecular main chain. Syndiotactic polypropylene was first disclosed by Natta et al. in U.S. Pat. No. 3,258,455. The Natta group obtained syndiotactic polypropylene by using a catalyst prepared from titanium trichloride and diethyl aluminum monochloride. A later patent to Natta et al., U.S. Pat. No. 3,305,538, discloses the use of vanadium triacetylacetonate or halogenated vanadium compounds in combination with organic aluminum compounds for producing syndiotactic polypropylene. U.S. Pat. No. 3,364,190 to Emrick discloses a catalyst system composed of finely divided titanium or vanadium trichloride, aluminum chloride, a trialkyl aluminum and a phosphorus-containing Lewis base as producing syndiotactic polypropylene. U.S. Pat. No. 4,892,851 disclosed a metallocene catalyst for producing highly crystalline syndiotactic polyolefins.
As disclosed in these patent references and as known in the art, the structure and properties of syndiotactic polypropylene differ significantly from those of isotactic polypropylene. The isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows: ##STR1##
Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" representing a "meso" dyad or successive methyl groups on the same side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.
In contrast to the isotactic structure, syndiotactic polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Using the Fischer projection formula, the structure of a syndiotactic polymer is designated as: ##STR2##
In NMR nomenclature, this pentad is described as . . . rrrr . . . in which each "r" represents a "racemic" dyad, i.e., successive methyl groups on alternate sides of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and, like the isotactic polymers, are insoluble in xylene.
This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer that is soluble in xylene. Atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product. While it is possible for a catalyst to produce all three types of polymer, it is desirable for a catalyst to produce isotactic or syndiotactic polymer with very little atactic polymer.
Catalysts that produce isotactic polyolefins are disclosed in European Patent Application Publication No. 284,708 corresponding to U.S. patent application Ser. No. 034,472 filed Apr. 3, 1987, and now abandoned; U.S. Pat. No. 4,794,096; and European Patent Application Publication No. 310,734 corresponding to U.S. patent application Ser. No. 095,755 filed on Sep. 11, 1987, and now abandoned. These applications disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of a highly isotactic polypropylene. The present invention, however, provides a different class of metallocene catalysts that are useful in the polymerization of syndiotactic/isotactic block polyolefins, and more particularly syndiotactic/isotactic block polypropylene.